A new engine designed in Germany reduces the pollutants in diesel exhaust emissions to barely measurable levels. The motor relies on extremely high fuel-injection and combustion pressures to burn fuel more completely--dramatically reducing both soot and nitrogen-oxide emissions.

Diesel engines use fuel more efficiently than gasoline engines and emit less carbon dioxide, but the trade-off is that they are usually more polluting. The higher combustion temperatures required to burn diesel lead to increased nitrogen-oxide emissions. And because diesel is heavy and less volatile than gasoline, not all the fuel is burned during combustion, resulting in the formation of soot particles. The worst offenders are buses and heavy-duty trucks.

Clean machine: Researchers at the Technical University-Munich have used this test engine to demonstrate the simultaneous reduction of nitrogen oxides and soot, without a catalytic converter.

Engineers at the Technical University-Munich (TUM) designed the new engine in a three-year project called Niedrigst-Emissions-LKW-Dieselmotor (NEMo), which translates to "lowest-emission diesel truck engine." Georg Wachtmeister, chair of internal combustion engines in the university's Department of Mechanical Engineering, led the effort. Using a single-cylinder research engine, Wachtmeister's team found a balance between exhaust gas recirculation, turboboost pressure, and fuel-injector nozzle configuration that allowed them to minimize both soot and nitrogen-oxide formation.

Modern diesel engines decrease nitrogen-oxide formation by cooling down part of their exhaust and recirculating it back into the combustion chamber (together with the fresh air used to burn the fuel). In this mixture, carbon dioxide and water from the exhaust gases moderate the combustion process, keeping the temperature in check. As a result, fewer nitrogen oxides are formed--but soot production increases, since the proportion of oxygen in the air-exhaust mixture is lower and the fuel burns less completely.

The TUM researchers designed their test engine so that the turbocharger compresses the air-exhaust mixture to 10 bar--roughly 10 times the atmospheric pressure at sea level--before introducing it into the combustion chamber. In contrast, mass-production engines can compress the mixture to a maximum of about 3.5 bar. Once compressed in this way, the air-exhaust mixture in the new engine contains enough oxygen for the diesel fuel to burn more completely. The maximum air pressure inside the combustion chamber is 300 bar, double that used in most production engines.

To offset the increased soot production caused by changing the exhaust-gas recirculation rate, the NEMo team modified the fuel-injector nozzle so that it atomizes diesel fuel at a pressure of over 3,000 bar, generating a fuel mist of microscopic particles that burns very quickly and practically soot-free. The most advanced production engines today use an injection pressure of about 1,800 bar.

With the modified exhaust-gas recirculation, boost pressure, and nozzle configuration, the TUM engine almost meets European emissions standards scheduled to take effect by 2014. Those standards stipulate that a heavy-truck diesel engine can emit only five milligrams of soot particles and 80 milligrams of nitrogen oxides per kilometer. Wachtmeister says that the TUM test engine met the nitrogen-oxide limits with "no problem" and is "very close" to the soot limits.

George Anitescu, a researcher at Syracuse University, is skeptical about the project's practicality. "The research may solve, somewhat, the trade-off between particulate matter and nitrogen-oxide formation" inherent to diesel combustion, he says. But he thinks the energy needed to achieve the high pressures used will decrease the engine's efficiency. Another concern, he says, is finding materials--particularly affordable ones--that can withstand the extreme pressures.

"For the time being, turning this design into a production engine is not practical," admits Wachtmeister. The TUM Internal Combustion Engines Workshop had to specially produce 95 of the components for the test engine. However, using these special components, the team was able to successfully apply the modifications to a production truck engine.

Wachtmeister expects that it will take between five and 10 years to come up with solutions that will allow the production of engines reliable enough to run for hundreds of thousands of kilometers without failing. The turbocharger and fuel-injection system will be particularly challenging to adapt for either heavy-duty trucks or car engines.

In the meantime, he says, the design could easily be implemented today in certain industrial engines such as diesel generators, the most common type used in standby and emergency power systems. And, Wachtmeister says, automotive companies in both Germany and Japan have expressed interest in the technology.